The walls of a typical industrial furnace used for the heat treating of metals are usually a composite structure made up of layers of firebrick, high- temperature insulation, steel plates, and outer surface cladding. Radiant tubes/beams and/or combustion burners inside the furnace provide the requisite heat. Consider a plane section of such a furnace wall where the inner wall surface is heated to 850°C by radiant heating from hot gases at 2000°C with a heat flux of 45,000 W/m². The convection heat transfer coefficient of the hot gases inside the furnace is estimated to be 15 W/m² K, the thermal conductance per unit area of the composite wall is 250 W/m² K, and there is convection at the outer surface of the furnace to ambient air at T∞. Sketch the thermal resistance circuit for the heat transfer in this system, and calculate the total heat flow per unit area through the composite wall and the exterior wall surface temperature. Also, if the outside air temperature is T∞ = 30°C, estimate the average convection heat transfer coefficient at the exterior surface.